For example, aluminum (Al) is known as a material of a wiring film used as an electrode of a thin-film transistor that is formed on an array substrate (active matrix substrate) of a liquid crystal display apparatus. Since an electric resistance of Al is very low, it has been generally used as a wiring material conventionally. In this specification, a film obtained not only from pure Al but also a material containing Al as a main components such as an Al alloy, and a wiring film resulted from patterning such a film are broadly referred to as an “Al film” and an “Al wiring film”, respectively.
A wet etching method using a chemical solution containing phosphoric acid and nitric acid series is common as a method of pattern process (patterning). However, since the method is isotropic etching, a sidewall of an Al wiring film resulted from patterning becomes substantially vertical. This causes an insulating film formed on the Al wiring film to have a poor step coverage characteristic, and, as a result, causes broken wiring of a wiring film formed on the insulating film, a reduction of a breakdown voltage of the insulating film, or the like.
A technique for etching the Al film in a tapered shape is proposed to solve such a problem in, for example, Japanese Patent Application Laid-Open Nos. 06-122982, 2001-77098, 2003-127397, and 2007-157755. However, when a composition of a chemical solution (etchant) changes as an etching process progresses or degrades by time in such a technique, an inclination of the sidewall of the tapered Al wiring film acutely changes accordingly. This poses a problem of a difficulty in managing the composition of the etchant.
According to the general wet etching technique for the Al film described above, the sidewall of the Al wiring film subjected to the patterning becomes substantially vertical. This causes the insulating film formed on the Al wiring film to have a poor step coverage characteristic, and, as a result, causes a reduction of a breakdown voltage of the insulating film, broken wiring of the wiring film of an upper layer, or the like, which eventually causes a drop in the yield of the product.
Japanese Patent Application Laid-Open Nos. 06-122982, 2001-77098, 2003-127397, and 2007-157755 propose an etching technique for forming the Al wiring film in a tapered shape. However, it is necessary to severely manage the composition of the etchant to obtain a predetermined tapered shape in a stable manner. To do so, it is necessary, for example, to introduce a facility to constantly monitor the concentration of the etchant, or replace the etchant more frequently, which eventually increases the cost.
Further, when an ordinary Al film is used as an electrode of a thin-film transistor (TFT) of an active matrix substrate for a liquid crystal display apparatus, contact characteristics between the TFT and a pixel electrode are drastically degraded due to an interface reaction with an ITO film which is a transparent pixel electrode. For this reason, it is difficult to apply the Al wiring film as a wiring film on the active matrix substrate.